Frequently, industrial applications require the transmission of electrical power and communication signals between a central command location and distant instrumentation and process control devices. Typically, full-duplex signal transmission is desired to provide instantaneous feedback to the central command location of process variables and actuation of process control equipment to enable more accurate regulation of the process. Usually, this requires a first pair of electrical conductors carrying commands from the central command location to a receiver local to the process control equipment and a second pair of conductors conveying instrumentation data back to the command location. These pairs of conductors are exposed to numerous causes of failure such as excessive stress placed on the conductors during operations, kinks which were placed in one or more of the conductors during their installation and which result in open or shorted circuits in one conductor pair, or failure of the couplings between the conductors and the local receiver. In applications requiring electrical transmission between a surface central command location and associated subsea process instrumentation and control equipment, for example, the transmission conductors are typically bound into a single umbilical cable connected to the local receiver by pairs of inductive couplers. The severity of a subsea environment can deleteriously affect the transmission paths, however, by causing the inductive coupler's insulation to fail thereby allowing seawater to enter the umbilical cable.
Regardless of the mode of failure, a loss of any one or pair of electrical conductors undesirably reduces the entire transmission network to a half-duplex mode, ultimately resulting in the loss of instrumentation feedback from the local receiver. The relative inaccessibility of the subsea portions of the transmission conductors, their bulk and their capital and installation costs prevent either quick replacement or restoration of the transmission network to full-duplex operation. Although transmission systems are available for providing full duplex communication over a single conductor pair (U.S. Pat. No. 4,136,327 of Flanders et al, for example), such systems require dedicated local receivers and interfaces between the local receivers and the conductors. Moreover, damage of a single conductor in such a full duplex conductor pair will immediately impair the integrity of the entire network.